In one conventionally known method of coding a speech signal, for example, with high efficiency, a sequence of samples of input original sound is divided into input signal sequences at regular intervals of about 5 to 50 ms, referred to as frames; a normalization value for the input signal sequence in each frame is obtained; the values of samples in the input signal sequence of each frame are normalized by the normalization value, and the resultant normalized input signal sequence is divided in accordance with a predetermined rule; and then vector quantization is performed.
A coding apparatus according to this coding method is shown in FIG. 27, and a decoding apparatus is shown in FIG. 28. The operation will be described briefly. The coding apparatus includes a normalization value generator 101, a normalizer 102, a divider 103, a vector quantizer 104, and a normalization value quantizer 105. The normalization value generator 101 receives an input signal sequence X={Xn; n=0 to N−1} that includes a plurality of samples of original sound in each frame, where N is the number of samples per frame. The input signal sequence X can be a sequence of samples in the time domain or a sequence of samples in a spectrum obtained by converting the sample sequence into the frequency domain in each frame. The normalization value generator 101 outputs a square root of a mean value of the powers of the samples in the input signal sequence X, as a normalization value G. The normalization value quantizer 105 quantizes the normalization value G obtained in the normalization value generator 101 and outputs a normalization value quantization index IG. In the case where the normalizer 102 uses a decoded normalization value G′ obtained by decoding the normalization value quantization index IG, the normalization value quantizer 105 outputs a decoded normalization value G′ as well.
The normalizer 102 receives the input signal sequence X and the normalization value G obtained by the normalization value generator 101 or the decoded normalization value G′ obtained by the normalization value quantizer 105, normalizes the input signal sequence by dividing each sample value (amplitude value) in the input signal sequence X by the normalization value G or the decoded normalization value G′ or by multiplying the sample value by the reciprocal of the normalization value G or the reciprocal of the decoded normalization value G′, and outputs a normalized input signal sequence x={xn; n=0 to N−1}. The divider 103 divides the normalized input signal sequence x output from the normalizer 102 to M divided input signal sequences ui (i=0 to M−1, where M is an integer greater than or equal to 1), in accordance with a predetermined rule, and outputs them. When M=1, no division is made, and x=u0, so that the divider 103 may be omitted.
The vector quantizer 104 performs vector quantization of each of the divided input signal sequences output from the divider 103 and outputs a vector quantization index ki. The vector quantizer 104 has a vector codebook 104T which associates a finite number of, for example, two or more, indexes with predetermined representative vector values, respectively. The vector quantizer 104 outputs an index ki corresponding to a representative vector value having the smallest distance measure to a given divided input signal sequence ui, as a vector quantization index.
The decoding apparatus includes a vector decoder 111, a reconstructing unit 112, a normalization value decoding unit 113, and an inverse normalizer 114. Like the vector quantizer 104 in the coding apparatus, the vector decoder 111 has a vector codebook 111T, decodes each vector quantization index ki given from the coding apparatus by reading out a representative vector value corresponding to ki from the vector codebook 111T, and outputs a divided output signal sequence vi. The reconstructing unit 112 reconstructs a normalized output signal sequence y by using the divided output signal sequence vi of a single frame given from the vector decoder 111, in accordance with a predetermined rule that equalizes the relationship between x and ui in the divider 103 of the coding apparatus and the relationship between y and vi. The normalization value decoding unit 113 decodes the normalization value quantization index IG sent from the coding apparatus and outputs a decoded normalization value G′. The inverse normalizer 114 receives the reconstructed normalization value output signal sequence y and the decoded normalization value G′, performs inverse normalization by multiplying the output signal sequence y by the decoded normalization value G′, and outputs an output signal sequence Y.
In the conventional coding method implemented by the coding apparatus and the decoding apparatus described above, variation in amplitude value among different input signal sequences of different frames can be reduced by normalizing the input signal beforehand in each frame, so that the efficiency of vector quantization can be improved. According to Patent literature 1, the frequency-domain signal of each frame is normalized, and the result is subjected to vector quantization.
Non-patent literature 1 indicates that, in CELP coding, each frame of the time-domain input signal is divided into subframes, and vector quantization is conducted on the powers of a series of the subframes.    Patent literature 1: Japanese Patent Application Laid Open No. H07-261800 (paragraphs [0016] to [0021])    Non-patent literature 1: Toshio Miki, et al., “Pitch Synchronous Innovation CELP (PSI-CELP),” the IEICE Transactions, Vol. J77-A, No. 3, pp. 314-324, March, 1994